I currently work as a Runner and i'd say it is one way to get into it. I've colleagues that have made the leap from behind the scenes to presenting. However, most of these people have acquired jobs after being approached and haven't set out for a career in presenting, they applied wanting to work in production. If you are genuinely interested in making good tv but then find yourself beind approached then i'd say go for it, but if you're only applying to 'get on the telly,' most production managers will see through this. I'd say if you know you want to be a presenter, get an agent!
OH and also, if you want to work in tv production, a broadcast journalism/media degree isn't necessary. I don't have a degree (yet) although i am going to university September. I've plenty of friends that started at 16 and worked their way up and are now really far into their game/equally many with non media related degrees who are doing just as well. Employers will look for enthusiasm, intelligence and hard work - a degree may help but won't be the be all end all. However, if you want to present news, i think you'll need a degree in broadcast journalism (i'm not 100% sure that's the right degree, but know you will need something journalism related.) If you want to work in weather presenting you'll need science related subjects. Hope this helps - good luck!